Secondary batteries such as a lithium ion secondary battery have been known as gradually deteriorating, thereby decreasing a battery capacity, due to repeated charge and discharge or even simple nonuse. Accordingly, there is heretofore disclosed a technique for ascertaining the degree of deterioration of a secondary battery and estimating a remaining battery life (see Patent Literature 1, for example).
For instance, a secondary battery mounted in a privately owned vehicle is often left unused for a longer period in the entire life, i.e., during vehicle parking, than in a charging/discharging state, i.e., during vehicle running. Accordingly, the battery life greatly results from the degree of progress of deterioration in an unused state. It is found that the progress of deterioration in the unused state largely depends on the temperatures during the unused period. Therefore, the temperature of a battery is conventionally measured (see Patent Literature 2, for example). A configuration for measuring the temperature of a battery appears as a temperature sensor in Patent Literature 1 and a built-in thermistor in Patent Literature 2.